Molecular oxygen is an essential nutrient for nonfacultative aerobic organisms, including, of course, humans. Oxygen is used in many important ways, namely, as the terminal electronic acceptor in oxidative phosphorylation, in many dioxygenase reactions, including the synthesis of prostaglandins and of vitamin A from carotenoids, in a host of hydroxylase reactions, including the formation and modification of steroid hormones, and in both the activation and the inactivation of xenobiotics, including carcinogens. The extensive P-450 system uses molecular oxygen in a host of important cellular reactions. In a similar vein, nature employs free radicals in a large variety of enzymic reactions.
Excessive concentrations of various forms of oxygen and of free radicals can have serious adverse effects on living systems, including the peroxidation of membrane lipids, the hydroxylation of nucleic acid bases, and the oxidation of sulfhydryl groups and of other sensitive moieties in proteins. If uncontrolled, mutations and cellular death result.
Biological antioxidants include well-defined enzymes, such as superoxide dismutase, catalase, selenium glutathione peroxidase, and phospholipid hydroperoxide glutathione peroxidase. Nonenzymatic biological antioxidants include tocopherols and tocotrienols, carotenoids, quinones, bilirubin, ascorbic acid, uric acid, and metal-binding proteins. Various antioxidants, being both lipid and water soluble, are found in all parts of cells and tissues, although each specific antioxidant often shows a characteristic distribution pattern. The so-called ovothiols, which are mercaptohistidine derivatives, also decompose peroxides nonenzymatically.
Free radicals, particularly free radicals derived from molecular oxygen, are believed to play a fundamental role in a wide variety of biological phenomena. In fact, it has been suggested that much of what is considered critical illness may involve oxygen radical ("oxyradical") pathophysiology (Zimmermen J. J. (1991) Chest 100: 189S). Oxyradical injury has been implicated in the pathogenesis of pulmonary oxygen toxicity, adult respiratory distress syndrome (ARDS), bronchopulmonary dysplasia, sepsis syndrome, and a variety of ischemia-reperfusion syndromes, including myocardial infarction, stroke, cardiopulmonary bypass, organ transplantation, necrotizing enterocolitis, acute renal tubular necrosis, and other disease. Oxyradicals can react with proteins, nucleic acids, lipids, and other biological macromolecules producing damage to cells and tissues, particularly in the critically ill patient.
Free radicals are atoms, ions, or molecules that contain an unpaired electron (Pryor, W. A. (1976) Free Radicals in Biol. 1: 1). Free radicals are usually unstable and exhibit short half-lives. Elemental oxygen is highly electronegative and readily accepts single electron transfers from cytochromes and other reduced cellular components; a portion of the O.sub.2 consumed by cells engaged in aerobic respiration is univalently reduced to superoxide radical (.O.sub.2.sup.-) (Cadenas E. (1989) Ann. Rev. Biochem. 58: 79). Sequential univalent reduction of .O.sub.2.sup.- produces hydrogen peroxide (H.sub.2 O.sub.2), hydroxyl radical (.OH), and water.
Free radicals can originate from many sources, including aerobic respiration, cytochrome P-450-catalyzed monooxygenation reactions of drugs and xenobiotics (e.g., trichloromethyl radicals, CCl.sub.3., formed from oxidation of carbon tetrachloride), and ionizing radiation. For example, when tissues are exposed to gamma radiation, most of the energy deposited in the cells is absorbed by water and results in scission of the oxygen-hydrogen covalent bonds in water, leaving a single electron on hydrogen and one on oxygen creating two radicals H. and .OH. The hydroxyl radical, .OH, is the most reactive radical known in chemistry. It reacts with biomolecules and sets off chain reactions and can interact with the purine or pyrimidine bases of nucleic acids. Indeed, radiation-induced carcinogenesis may be initiated by free radical damage (Breimer L. H. (1988) Brit. J. Cancer 57: 6). Also for example, the "oxidative burst" of activated neutrophils produces abundant superoxide radical, which is believed to be an essential factor in producing the cytotoxic effect of activated neutrophils. Reperfusion of ischemic tissues also produces large concentrations of oxyradicals, typically superoxide (Gutteridge J. M. C. and Halliwell B. (1990) Arch. Biochem. Biophys. 283: 223). Moreover, superoxide may be produced physiologically by endothelial cells for reaction with nitric oxide, a physiological regulator, forming peroxynitrite, ONOO.sup.- which may decay and give rise to hydroxyl radical, .OH (Marletta, M. A. (1989) Trends Biochem. Sci. 14: 488; Moncada et al. (1989) Biochem. Pharmacol. 38: 1709; Saran et al. (1990) Free Rad. Res. Commun. 10: 221; Beckman et al. (1990) Proc. Natl. Acad. Sci. (U.S.A.) 87: 1620). Additional sources of oxyradicals are "leakage" of electrons from disrupted mitochondrial or endoplasmic reticular electron transport chains, prostaglandin synthesis, oxidation of catecholamines, and platelet activation.
Many free radical reactions are highly damaging to cellular components; they crosslink proteins, mutagenize DNA, and peroxidize lipids. Once formed, free radicals can interact to produce other free radicals and non-radical oxidants such as singlet oxygen (.sup.1 O2) and peroxides. Degradation of some of the products of free radical reactions can also generate potentially damaging chemical species. For example, malondialdehyde is a reaction product of peroxidized lipids that reacts with virtually any amine-containing molecule. Oxygen free radicals also cause oxidative modification of proteins (Stadtman E. R. (1992) Science 257: 1220).
Aerobic cells generally contain a number of defenses against the deleterious effects of oxyradicals and their reaction products. Superoxide dismutases (SODs) catalyze the reaction: EQU 2.O.sub.2.sup.- +2H.sup.+ .fwdarw.O.sub.2 +H.sub.2 O.sub.2
which removes superoxide and forms hydrogen peroxide. H.sub.2 O.sub.2 is not a radical, but it is toxic to cells; it is removed by the enzymatic activities of catalase and glutathione peroxidase (GSH-Px). Catalase catalyzes the reaction: EQU 2H.sub.2 O.sub.2 .fwdarw.2H.sub.2 O+O.sub.2
and GSH-Px removes hydrogen peroxide by using it to oxidize reduced glutathione (GSH) into oxidized glutathione (GSSG) according to the following reaction: EQU 2 GSH+H.sub.2 O.sub.2 .fwdarw.GSSG+2H.sub.2 O
Other enzymes, such as phospholipid hydroperoxide glutathione peroxidase (PLOOH-GSH-Px), converts reactive phospholipid hydroperoxides, free fatty acid hydroperoxides, and cholesterol hydroperoxides to corresponding harmless fatty acid alcohols. Glutathione S-transferases also participate in detoxifying organic peroxides. In the absence of these enzymes and in presence of transition metals, such as iron or copper, superoxide and hydrogen peroxide can participate in the following reactions which generate the extremely reactive hydroxyl radical .OH.sup.- : EQU .O.sub.2.sup.- +Fe.sup.3+ .fwdarw.O.sub.2 +Fe.sup.2+ EQU H.sub.2 O.sub.2 +Fe.sup.2+ .fwdarw..OH+OH.sup.- +Fe.sup.3+
In addition to enzymatic detoxification of free radicals and oxidant species, a variety of low molecular weight antioxidants such as glutathione, ascorbate, tocopherol, ubiquinone, bilirubin, and uric acid serve as naturally-occurring physiological antioxidants (Krinsky N. I. (1992) Proc. Soc. Exp. Biol. Med. 200:248-54). Carotenoids are another class of small molecule antioxidants and have been implicated as protective agents against oxidative stress and chronic diseases. Canfield et al. (1992) Proc. Soc. Exp. Biol. Med. 200:260 summarize reported relationships between carotenoids and various chronic diseases, including coronary heart disease, cataract, and cancer. Carotenoids dramatically reduce the incidence of certain premalignant conditions, such as leukoplakia, in some patients.
In an effort to prevent the damaging effects of oxyradical formation during reoxygenation of ischemic tissues, a variety of antioxidants have been used.
One strategy for preventing oxyradical-induced damage is to inhibit the formation of oxyradicals such as superoxide. Iron ion chelators, such as desferrioxamine (also called deferoxamine or Desferol) and others, inhibit iron ion-dependent .OH generation and thus act as inhibitors of free radical formation (Gutteridge et al. (1979) Biochem. J. 184: 469; Halliwell B. (1989) Free Radical Biol. Med. 645; Van der Kraaij et al. (1989) Circulation 80: 158). Amino-steroid-based antioxidants such as the 21-aminosteroids termed "lazaroids" (e.g., U74006F) have also been proposed as inhibitors of oxyradical formation. Desferrioxamine, allopurinol, and other pyrazolopyrimidines such as oxypurinol, have also been tested for preventing oxyradical formation in a myocardial stunning model system (Bolli et al. (1989) Circ. Res. 65: 607) and following hemorrhagic and endotoxic shock (DeGaravilla et al. (1992) Drug Devel. Res. 25: 139). However, each of these compounds has notable drawbacks for therapeutic usage. For example, deferoxamine is not an ideal iron chelator and its cellular penetration is quite limited.
Another strategy for preventing oxyradical-induced damage is to catalytically remove oxyradicals such as superoxide once they have been formed. Superoxide dismutase and catalase have been extensively explored, with some success, as protective agents when added to reperfusates in many types of experiments or when added pre-ischemia (reviewed in Gutteridge J. M. C. and Halliwell B. (1990) op.cit.). The availability of recombinant superoxide dismutase has allowed more extensive evaluation of the effect of administering SOD in the treatment or prevention of various medical conditions including reperfusion injury of the brain and spinal cord (Uyama et al. (1990) Free Radic. Biol. Med. 265; Lim et al. (1986) Ann. Thorac. Surg. 42: 282), endotoxemia (Schneider et al. (1990) Circ. Shock 30: 97; Schneider et al. (1989) Prog. Clin. Biol. Res. 308: 913, and myocardial infarction (Patel et al. (1990) Am. J. Physiol. 258: H369; Mehta et al. (1989) Am. J. Physiol. 257: H1240; Nejima et al. (1989) Circulation 79: 143; Fincke et al. (1988) Arzneimittelforschung 38: 138; Ambrosio et al. (1987) Circulation 75: 282), and for osteoarthritis and intestinal ischemia (Vohra et al. (1989) J. Pediatr. Surg. 24: 893; Flohe L. (1988) Mol. Cell. Biochem. 84: 123). Superoxide dismutase also has been reported to have positive effects in treating systemic lupus erythematosus, Crohn's disease, gastric ulcers, oxygen toxicity, burned patients, renal failure attendant to transplantation, and herpes simplex infection.
An alternative strategy for preventing oxyradical-induced damage is to scavenge oxyradicals such as superoxide once these have been formed, typically by employing small molecule scavengers which act stoichiometrically rather than catalytically. Congeners of glutathione have been used in various animal models to attenuate oxyradical injury. For example, N-2-mercaptopropionylglycine has been found to confer protective effects in a canine model of myocardial ischemia and reperfusion (Mitsos et al. (1986) Circulation 73: 1077) and N-acetylcysteine ("Mucomyst") has been used to treat endotoxin toxicity in sheep (Bernard et al. (1984) J. Clin. Invest. 73: 1772). Dimethyl thiourea (DMTU) and butyl-.alpha.-ephenylnitrone (BPN) are believed to scavenge the hydroxyl radical, .OH, and has been shown to reduce ischemia-reperfusion injury in rat myocardium and in rabbits (Vander Heide et al. (1987) J. Mol. Cell. Cardiol. 19: 615; Kennedy et al. (1987) J. Appl. Physiol. 63: 2426). Mannitol has also been used as a free radical scavenger to reduce organ injury during reoxygenation (Fox R. B. (1984) J. Clin. Invest. 74: 1456; Ouriel et al. (1985) Circulation 72: 254).
Thus, application of inhibitors of oxyradical formation and/or enzymes that remove superoxide and hydrogen peroxide and/or small molecule oxyradical scavengers have all shown promise for preventing reoxygenation damage present in a variety of ischemic pathological states and for treating or preventing various disease states associated with free radicals. However, each of these categories contains several drawbacks. For example, inhibitors of oxyradical formation typically chelate transition metals which are used in essential enzymatic processes in normal physiology and respiration; moreover, even at very high doses, these inhibitors do not completely prevent oxyradical formation. Superoxide dismutases and catalase are large polypeptides which are expensive to manufacture, do not penetrate cells or the blood-brain barrier, and generally require parenteral routes of administration. Free radical scavengers act stoichiometrically and are thus easily depleted and must be administered in high dosages to be effective.
Based on the foregoing, it is clear that a need exists for antioxidant agents which are efficient at removing dangerous oxyradicals, particularly superoxide and hydrogen peroxide, and which are inexpensive to manufacture, stable, and possess advantageous pharmacokinetic properties, such as the ability to cross the blood-brain barrier and penetrate tissues. Such versatile antioxidants would find use as pharmaceuticals, chemoprotectants, and possibly as dietary supplements. It is one object of the invention to provide a class of novel antioxidants which possess advantageous pharmacologic properties and which catalytically and/or stoichiometrically remove superoxide and/or hydrogen peroxide.
The references discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate such disclosure by virtue of prior invention. All publications cited are incorporated herein by reference.